Activation of the Immune System:
The immune system is capable of killing autologous cells when they become infected by virus or when they transform into cancer cells. Such a potentially dangerous mechanism is under tight control. When they have not yet encountered their specific antigen, the immune system's T-killer cells (CTL) circulate as inactive precursors. To be activated, the precursor T-killer cell must recognize its specific antigen peptide, presented by MHC class I molecules on professional antigen presenting cells (APC). This antigen specific cellular interaction is, however, not enough to fully activate the CTL, notwithstanding the co-stimulatory signals from the APC.
Until recently it was believed that a T-helper cell that recognises the same antigen on the same APC as the CTL is needed to fully activate the CTL. Upon activation, the specific T-helper cell would supply cytokines such as IL-2 needed for the activation of the CTL. Guerder and Matzinger (J. Exp. Med. 176:553 (1992)), however, proposed the “licensing” model for CTL activation. In this model it was suggested that the T-helper cell, when recognising its antigen on a professional APC, would deliver an activation signal to the APC that as a result would be able to subsequently activate a CTL without the need for the T-helper cell to be present. Recently, the molecular mechanism of the licensing model was elucidated. Schoenberger et al. (Nature 393:480 (1998)), described the crucial role of the CD40L-CD40 pathway in the licensing model. Activation of the T-helper cell by the dendritic cell (DC) results in the up-regulation of the CD40L, which subsequently provides the signal that empowers the DC for CTL priming by triggering the CD40 molecule on the DC.
DC circulate through and are resident in the body tissues and at sites of antigen deposition or introduction. After taking up antigens, they migrate to the draining lymph nodes where they present antigen to the T cells. It is well known that a DC needs to be activated to perform optimally. Resting DC express only low levels of MHC and co-stimulatory molecules and are poor stimulators of T cells. DC can be activated by inflammatory cytokines and bacterial products, which results in up-regulation of MHC and co-stimulatory molecules. Therefore, DC that have encountered antigens under inflammatory conditions will readily activate T-helper cells when they arrive in the draining lymph nodes. It is thus very likely that the combination of inflammatory cytokines at the site of antigen uptake and the CD40L-CD40 interaction during the T-helper cell interaction result in an optimal capacity to license the DC for CTL activation.
The CD40 Molecule and the TNF Receptor Family:
The CD40 molecule belongs to the TNF receptor family of type I transmembrane proteins. The members of this gene family (which include among others, the two receptors for TNF, the low-affinity nerve growth factor receptor and the T cell activation antigen CD27, CD30, and CD95) are characterized by sequence homology in their cysteine-rich extracellular domains (Armitage et al., Current Opinion in Immunology 6:407 (1994)). The known ligands for the members of the TNF receptor family are homologous as well. Although TNF-α is a soluble cytokine, it is initially synthesized as a membrane associated molecule. Most of the members of the TNF/CD40L receptor and the TNF/CD40 families are type II trans-membrane proteins. These include: hTNF-α, hLT, hLT-β, hCD40L, hCD27L, hCD30L, cfECP1, myx VRh, mCD30, hCD27, hFas, m4-1BB, rOX-40, hTNFR-h, hTNFR-II, hTNFR-1 and hLNGFR. CD40 is best known for its function in B-cell activation. The molecule is constitutively expressed on all B cells. CD40L-CD40 interaction can stimulate the proliferation of purified B cells and, in combination with cytokines, mediate immunoglobulin production. Recent studies indicate that the distribution of the CD40 molecule is not as restricted as was originally postulated. Freshly isolated human monocytes express low levels of the CD40 molecule, which can be up-regulated by culturing them in the presence of IFN-α (Alderson et al., J. Exp. Med. 178:669 (1993)). Stimulation of monocytes via CD40 results in the secretion of pro-inflammatory cytokines such as IL-1 and TNF-α, toxic free radical intermediates such as nitric oxide and up-regulation of the B7 co-stimulatory molecules. Human DC isolated from peripheral blood can also express the CD40 molecule (Caux et al., J. Exp. Med. 180:263 (1994)). Ligation of CD40 on DC results in enhanced survival of these cells when cultured in vitro. As with monocytes, stimulation of DC via CD40 results in secretion of pro-inflammatory cytokines such as IL-12 and TNF-α and up-regulation of the CD80/86 co-stimulatory molecules. In addition, it was recently demonstrated that activation of CD40 induces the capacity to stimulate the activation of killer T cells (Schoenberger et al., Nature 393:480 (1998)). Accordingly, activating CD40 by binding it with a ligand, such as an antibody, would induce a number of humoral and cytotoxic effects, useful in inhibiting tumors.